This invention relates to unmanned underwater vehicles and more particularly to the control thereof so as to operate substantially in a hovering mode. Certain unmanned underwater vehicles, for example carrying scientific or other instrumentation, have a requirement to operate at two or more depths and to maintain the ordered depth, within certain tolerances for a specified period of time.
Among the approaches used heretofore for obtaining hovering control are propellors driven by variable speed motors, alternation of intermittent periods of propellor driven ascent and drogue delayed descent as shown in U.S. Pat. No. 4,007,505, by controlling discharge of heavy and light liquid to increase or decrease buoyancy as shown in U.S. Pat. No. 3,228,369, and by flooding and blowing of chambers as shown in U.S. Pat. No. 3,180,297.
In addition, it is known to increase buoyancy of an underwater vehicle by discharge or dropping of weights, often in the form of shot, as shown in U.S. Pat. No. 3,004,507. Since the dropping of a weight is an irreversible action, this expedient has generally been confined to a one time event, such as for the recovery of a spent practice torpedo at the end of its run, rather than for effecting hovering control at an assigned depth.
The mentioned known hovering systems have a variety of drawbacks or shortcomings when applied to an expendable unmanned instrumentation vehicle that is required to be stored for considerable period of time and then deployed and required to operate reliably for a predetermined, relatively short period of time. Then shortcomings include complex and expensive controls, noise of motors and pumps, requirement of substantial energy storage, size constraints, and limited reliability after prolonged periods of non-use.